1. Field of the Invention
This invention relates to an automatic transmission fluid composition, and more particularly to an automatic transmission fluid composition which is excellent in shift feeling as well as prevention of slip after contacting the clutch, and in addition which has a short shift time and a very small change in the friction coefficient during use.
2. Background Art
An automatic transmission is a transmission having a mechanism in which the transmission torque ratio is automatically established in response to the speed of the car, magnitude of load or the like. Such an automatic transmission comprises a torque converter part, a multiple disk clutch, a planetary gear part, and a hydraulic controlling part. The automatic transmission fluid is a common lubricating oil for all the parts described above, and which functions as a transfer fluid for motive power, as a lubricating oil for gears and bearings, and as a hydraulic fluid for controlling devices.
A shift operation wherein various reduction gear ratios are attained by changing the connections between the respective members of planetary gears is carried out by utilizing clutches and a braking band. At the present time, the general type of clutch is a wet clutch composed of a multiple disk clutch comprising a driven plate of steel and a drive plate of a paper frictional material. The frictional properties of such a wet clutch greatly influence the transmitting function of the automatic transmission unit, and in turn, the shift feeling of the motorcar.
While frictional properties of the wet clutch change depending upon the combination of the paper frictional material being the drive plate and the automatic transmission fluid, the influence due to composition of the automatic transmission fluid is remarkable; therefore, in recent years, in view of smoother shift feeling, there is a tendency to lay stress on the frictional properties of automatic transmission fluid in wet clutch, and a need exists for an automatic transmission fluid having good frictional characteristics.
Such frictional properties are evaluated by a SAE No. 2 friction tester, which is well known by those skilled in the art. This type of tester is essentially an inertia dynamometer wherein the kinetic energy of a rotor is spent by a frictional plate, and a coefficient (.mu.) of friction is calculated from the friction torque.
As methods for measuring coefficient of friction, there are the dynamic method and the static method. The dynamic method is effected in such a manner that an inertia plate (mounted on a motor shaft) is rotated at a high speed by means of a motor for a given period of time, thereafter the motor power supply is shut off, and at the same time pneumatic pressure is applied to a piston, whereby the driven plate is pressed against the drive plate which has been rotated integrally with the motor shaft to stop the movement of the inertia plate. A dynamic friction coefficient (.mu.d) at the time of rotating the motor is obtained from the friction torque curve in the above case. Furthermore a friction coefficient at the end of clutch engaging (.mu.o) is obtained from the friction torque immediately before the stop.
In the static method, the friction torque produced by a facing up of the drive plate and the driven plate is obtained by rotating the inertia plate by means of an auxiliary motor at a very low speed while applying pneumatic pressure to the piston. The static breakaway friction coefficient (.mu.s) is calculated from the value obtained as described above.
In general, a larger value of .mu.d is desired for a short shift time, and a larger value of .mu.s is desired for more effective prevention of slip after contacting the clutch. On the other hand, a value of .mu.o/.mu.d closer to 1.0 provides a smoother shift feeling, and is thus desirable. In recent years, there has been a demand for an automatic transmission fluid having a much higher value of both the coefficients of friction (.mu.d and .mu.s) because such a fluid leads to motorcars equipped with lightweight and small-sized transmission clutches, and such cars will give better fuel economy.
However, presently available automatic transmission fluids are less than satisfactory for good frictional properties.